Apple released the 3.1 beta firmware and SDK to developers, which seems to include a few new features such as:
Non-destructive video editing
Voice Control over Bluetooth
iPhone vibrates when moving icons
New APIs to enable third party apps to access videos and edit them
“Fraud Protection” toggle
Faster boot time
The biggest change is the update of the baseband to 5.08.01.
If you don’t care about jailbreaking or unlocking your iPhone, then this doesn’t matter to you. However, if you want to keep you device jailbroken/unlocked, you should not update to 3.1 beta, or even the official 3.1 firmware when it’s released to the general public.
Like the Dev Team warned us, updating to 3.1 will prevent you from running any jailbreak method, which would make it impossible to unlock.
Ultrasn0w users must stay away from any firmware updates past 3.0 (including today’s 3.1 beta) until we release the tools that let you update the firmware without updating the baseband. For most phones out there, baseband updates are irreversible and you’ll lose ultrasn0w.
This warning does not apply to the iPhone 2G, which uses BootNeuter for the unlock, not ultrasn0w.
My advice is to wait and see. The Dev Team is most likely already picking the 3.1 beta apart. Stay tuned for more updates as they become available.